Prostheses of this type are used with ball-and-socket joints, particularly with shoulder and hip joints. Fundamentally there are with prostheses for ball-and-socket joints two different solution versions.
In a first version, the ball is entirely or at least half detached and replaced by a prosthesis, with this having a shaft that is inserted into the remaining bone or its medullary space. Such shaft prostheses are especially suited for older persons and/or in a case of bone fractures, for example femoral neck fractures.
With a second version, merely the surface of the bone or cartilage is at least partially replaced. One speaks in this case of “surface replacement prostheses,” “resurfacing prostheses,” or also “cap prostheses.” Such prostheses are for example offered under the trade names “Durom” and “Copeland.” These prosthesis types have the advantage that their implantation is minimally invasive. The duration of operation is shorter. For a replacement of the prosthesis, there are more possibilities. In particular a reverse operation is offered, in which the prosthesis is replaced by a shaft prosthesis, as it is described above.
One problem with surface replacement prostheses however consists in that their attachment mostly is harder than in prostheses with a shaft. There are various approaches to implement the attachment.
Thus there are surface replacement prostheses which consist only of a cap, but for this in essence they surround the entire joint ball. According to appearances this at least leads to a better distribution of forces, however more bone material must be removed and the stability to tensile forces is imperfect under certain circumstances.
Additionally there are surface prostheses that have a handle, whereby its length corresponds roughly to the ball diameter. Thereby the forces are distributed to additional contact surfaces and the prosthesis is better centered on the joint ball. However the solution is imperfect, among other things because the handle is subject to lever forces and thus under certain circumstances can lead to a loosening of the prosthesis.
In some cases also with the known prosthesis the problem is posed that the bone necrotizes in the area of the joint head, and thus particularly at the contact surfaces with the prosthesis,
U.S. Pat. No. 4,328,593 shows a surface prosthesis with a cap that is larger than a hemisphere and has a cylindrical, hollow handle that projects far over the lower edge of the cap. U.S. Pat. No. 5,549,704 shows a surface prosthesis similar to the named US document. FR-A-2 578 739 shows a surface prosthesis in which a part of the cap is smaller than a hemisphere and a flap part projects over the cap. For attachment a threaded bolt is provided into which a peg of the cap engages. FR-A-2 737 970 shows a surface prosthesis with a cap that is smaller or larger than a hemisphere and which has a central handle.